The radio frequency spectrum is a rare resource for mobile communication. Spatial diversity allows increasing data rates of a cellular network by increasing the density of intra-frequency cells. The transmission power of the cells may also be adapted to indoor and outdoor environments, e.g., in heterogeneous networks (also abbreviated by HetNet), which have been introduced in Release 9 of Long Term Evolution (LTE). Out of the plurality of at least pairwise overlapping cells, a mobile terminal or mobile station, which is generally referred to as User Equipment (UE), has to detect a suitable cell for cell selection and reselection in an idle mode and for preparing handovers in a connected mode.
Cell synchronization is a very first step when the UE wants to camp on a cell of the cellular network. The UE acquires a Physical Cell Identity (PCI) of the cell and a radio frame timing, which information enables the UE to read the Master Information Block (MIB) and one or more System Information Blocks (SIB).
The UE reads a Primary Synchronization Signal (PSS) indicative of a physical layer identity and a Secondary Synchronization Signal (SSS) indicative of a physical layer cell identity group number. The PCI is the combination of the physical layer identity and the physical layer cell identity group number.
The physical layer identity has only three different hypotheses, so that the UE may receive time-shifted copies of the PSS indicative of the same physical layer identity from different cells. Even if the UE would consider only the two strongest cells, the three different values possible for the physical layer identity are not sufficient to distinguish neighboring cells as a consequence of the Four Color Theorem. Collisions between different intra-frequency cells having the same physical layer identity can also occur in heterogeneous networks including a picocell embedded within a macrocell, or in dense cellular topologies including areas covered by three or more different cells.
As a consequence, when the UE perform interference cancellation by canceling the PSS of the interfering cell in order to detect one or more other cells, the PSS of all cells having the same physical layer identity is removed from the received signal, so that the UE cannot detect any further cells having the same physical layer identity as the strongest cell.